Active and highly durable supported catalysts for proton exchange membrane electrolysers

The design and development of supported catalysts for the oxygen evolution reaction (OER) is a promising pathway to reducing iridium loading in proton exchange membrane water electrolysers. However, supported catalysts often suffer from poor activity and durability, particularly when deployed in membrane electrode assemblies. In this work, we deploy iridium coated hollow titanium dioxide particles as OER catalysts to achieve higher Ir mass activities than the leading commercial catalysts. Critically, we demonstrate state-of-the-art durabilities for supported iridium catalysts when compared against the previously reported values for analogous device architectures, operating conditions and accelerated stress test profiles. Through extensive materials characterisations alongside rotating disk electrode measurements, we investigate the role of conductivity, morphology, oxidation state and crystallinity on the OER electrochemical performance. Our work highlights a new supported catalyst design that unlocks high-performance OER activity and durability in commercially relevant testing configurations.

Where E RHE is potential vs. RHE, E WE is the measured potential of the working electrode (WE) against Hg/HgSO 4 reference and E REF is the standard potential of the Hg/HgSO 4 reference electrode (0.642 V).

Calculation of mass specific activity
The current can be normalised to the theoretical mass of Ir in each synthesised powder to obtain Ir-mass activity (I m , A/g Ir ).For the commercial standard, IrO 2 , the mass activity was calculated based on the known mass of Ir within the IrO 2 rutile structure.Utilising the area of the working electrode (0.196 cm 2 ) and the mass of Ir, the Ir-mass activity was calculated as shown below if current densities (I d ) are known.The theoretical iridium loading onto the RDE was 5 x10 -6 g Ir for all electrochemical experiments.

Equation. S4
Electronic Supplementary Material (ESI) for EES Catalysis.This journal is © The Royal Society of Chemistry 2024    S2.              solution nanocatalysts with enhanced activity and stability for oxygen evolution, J. Power Sources, 2016, 325, 15-24.

Figure
Figure S1: a) Electrochemical impedance spectroscopy (EIS) plot of supported catalysts at 100 mHz -200 kHz from three-electrode system b) Enlarged view of the ohmic resistance from the EIS plot.Dashed lines indicate the model fit.

Figure S2 :
Figure S2: Current density (geometric) comparison of various literature Ir catalysts supported on or integrated into titanium containing metal oxide supports.50-TiO 2 -H 2 , 50-WH1-H 2 and 50-WH5-H 2 from this work are also plotted (as lines).The literature references are shown in the supporting information as refs. 2 -15.Symbols indicate Ir-TiO 2 catalysts only (circle) and Ir-Modified TiO 2 (square), modified relates to the addition of either metal or non-metal elements to Ti and the colour of the symbols relates to Ir wt%.The literature data is listed in further detail below in TableS2.

Figure S3 :
Figure S3: Stability comparison of cycle 1 and cycle 10 for supported catalysts a) thermally reduced and b) thermally oxidised.All experiments were conducted in 0.1 M HClO 4 electrolyte with a Au disk working electrode.Electrochemical analysis was assessed on the 1 st and 10 th CV cycle with theoretical Ir loading of 25.5 µg Ir /cm 2 .Note the differences in the magnitude of the Y-axes for A and B.

Figure S4 :
Figure S4: Tafel plots for the supported Ir catalysts with the calculated Tafel slopes.

Figure S5 :
Figure S5: XRD diffraction pattern of a) supports without Ir and b) thermally oxidised TiO 2 supported Ir catalysts including reference patterns with the corresponding ICSD numbers.

Figure S6 :
Figure S6: Characterisation of WH1 and WH5 AuPd-TiO 2 supports as synthesised (no annealing).a) HAADF-STEM image, b-d) STEM-EDX elemental maps and e) High resolution TEM image and f) summed EDS spectra for the region shown for WH1.g) HAADF STEM image, h-j) STEM-EDX elemental maps and k) summed STEM-EDX spectra for the region shown for WH5.Yellow box in (e) highlights the lattice fringe analysis region with a measured lattice spacing of 0.2 nm corresponding to the {111} spacing of AuPd alloys.

Figure S7 :
Figure S7: Particle diameter distribution obtained from (a) 67 and 72 measurements of AuPd nanoparticle size and (b) 28 and 23 measurements of the porous TiO 2 supports in WH1 and WH5, respectively.A Mann Whitney U test was used to compare the mean particle diameter between WH1 and WH5 in a) AuPd and b) TiO 2 supports.There was no significant difference between WH1 and WH5 particle size means for AuPd (p = 0.27) or for TiO 2 (p = 0.59).

Figure S12 :
Figure S12: XPS spectra of the synthesised supported catalysts for a) Au 4f and b) Ti 2p regions.

Figure S14 :
Figure S14: XPS spectra for a) Ir 4f, b) Au 4f and c) Ti 2p regions of the 50-WH1-H 2 and 50-WH5-H 2 in the form of nanopowder and post 10k AST catalyst coated membranes (EoL).Grey dashed lines are provided to guide the eye.The nanopowder spectra are taken from Figures 5b and S12.
Figure S14: XPS spectra for a) Ir 4f, b) Au 4f and c) Ti 2p regions of the 50-WH1-H 2 and 50-WH5-H 2 in the form of nanopowder and post 10k AST catalyst coated membranes (EoL).Grey dashed lines are provided to guide the eye.The nanopowder spectra are taken from Figures 5b and S12.

Table S1 :
Mass of gold and palladium deposited on TiO 2 supports determined by ICP-OES.

Table S2 :
Comparison of the OER activities from rotating disk electrode from the literature with mass activities and geometric area normalised current densities where available.

Table S3 :
Average (mean ± SD) AuPd nanoparticle and TiO 2 porous sphere diameters in the WH1 and WH5 catalyst-supports (prior to Ir deposition) determined from TEM and SEM.